Yesterday Dell announced its quad 7800GTX SLI motherboard, but is such a monstrosity something to really value, or is it the ultimate White Elephant?

When Michael Dell and Jen-Hsun Huang stood up in front of CES together
and announced the Dell XPS 600 Renegade, a product manager for a major
motherboard manufacturer began to laugh hysterically. Dual
7800GTX video cards are nothing new -- I was shown several prototypes
back in October. Putting two of those cards in an SLI
configuration is something that has been feasible for several months,
but only on beta or severely hacked drivers. NVIDIA's partnership
with Dell means they will now need to provide quad SLI support inside
the core Detonator drivers, or release specific drivers for these
systems. The first option leaves the possibility for some
interesting driver hacks, the second option might not be practical
depending on the actual volume of these systems.

But pragmatically looking at the performance of such a system, what can
we really expect? Many modern games only receive a 40%
performance boost by putting 7800GT cards into dual SLI mode
at modest resolutions, like 2048x1536. Dual 7800GTX cards receive
only about a 33% performance boost in the same benchmarks, but out of
the gate a single 7800GTX out performs two 7800GT cards in SLI.

Diminishing returns is already apparent with only two high
performance 7800GTX cards, and there are similar results elsewhere for
7800GTX 512MB cards. Although there is no claim anywhere that
this new XPS Renegade will be a mainstream system, hopefully we see
more innovative and efficient (maybe even quiet) systems from the
world's largest PC manufacturer, rather than another dirty hack to grab
headlines... Fortunately Dell just released their 3007FPW to calm our tensions.

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I don't think you experience diminishing returns but rather become limited by the CPU. I'm not saying that you'll experience 100% speed ups by adding a second adapter but something like 80% or so is quite possible unless the SLI bridge is becoming a bottle neck.

If a Dual 7800GTX only sees a 33% increase, but costs 100% more over a 7800GTX, and a Dual 7800GTX in SLI mode offers another extrapolated 30% on top of that for another doubling in cost, is that not the very definition of "diminishing returns"?

$x for y performance
$2x for 1.33y performance
$4x for 1.77y performance
...
Logically then, it would continue to $8x for 2.1y performance

Diminishing returns implies that you lose efficiency. I'm not saying that going from 2 card SLI to 4 card SLI will give you the same return or that the efficiency will be the same, but rather that using values under which they are clearly CPU limited and explaining that as diminishing returns is not correct. In truth the performance benefit potential from going SLI is somewhere between 80-90% (with 2 cards, dunno about 4) but with the most powerful cards your CPU becomes a huge bottleneck (heck, an FX57 may even bottleneck a single GTX7800 512 in some programs). Sure there is no CPU to match the rendering power provided by these setups but claiming that these dont perform much better than 1 card systems is due to diminishing returns is akin throwing a low FSB on a high clock P4 and then claiming that performance bites because clockspeed doesn't help much at all.

The law of diminishing returns holds that, holding everything else constant, adding more gives you less. This applies, for example, with eating cheeseburgers. You get quite a bit of utility from one cheeseburger, and perhaps nearly as much from a second, but almost nothing from a third, and possibly even negative return on a fourth (upset stomach or vomiting, for example).

So in this case adding on 7800GTX is awesome because it outperforms SLI 7800GT, which itself is 40% better than a single 7800GT; the point of diminishing returns hits early though, because adding a second 7800GTX in the form of a Dual 7800GTX only gives you a 33% improvement over the 7800GTX, and therefore in theory adding two more 7800GTXs, in the form of a Dual 7800GTX will probably add little exactly because they are CPU, memory, or bus limited, which is the very meaning of the term "diminishing returns".

If you can rejigger the busses, or the CPU, or the ram so that adding a Dual 7800GTX improves performance another 45%, then you have broken out of "diminishing returns" and into the land of "significant improvement". But since we aren't changing the CPU, bus, or ram when we add another Dual 7800GTX, we live squarely in the land of diminishing returns.

Now that I reread your post I'm actually a bit offended by your saying that I "lack logic"... First off, in this comparison diminishing returns has nothing to do with the price. Nothing at all. Secondly, having a second GeForce GTX "add" 30% does not imply that the returns of adding more cards will be less, since you only have two points to work with. It could be that drivers happen to like odd numbers better or a thousand different things.

Regardless of this, why does having two GeForce 6600GTs in SLI gives you a better "improvement" over having two 7800GTXs in SLI? If you but think about it you'll notice that this has nothing to do with a huge decrease in marginal utility capacity but, rather, has to do with the fact that the processor cannot feed the video cards with enough work to maximize the "rendering" capacity of both GPUs.

quote:First off, in this comparison diminishing returns has nothing to do with the price. Nothing at all

Maybe you don't understand what diminishing returns means. It's the amount of performance you get vs how much you are paying. So price has everything to do with it. And even though we only really have 2 points to reference, you can with a high degree of certainty use other points of reference to judge how this system should perform. Namely, if you know how much video computational power you have (Using cards other than a 7800GTX), you can clearly see that the framerate does not scale linerally with the amount of video processing power you have once you reach a certain threshold, and that threshold is reached long before Quad-SLI in every game out on the marketplace today. While you are correct in that yes, given you were to plug this into a P4 running at 10GHz, it MAY be worthwhile, it's not. Therefore running Quad-SLI in the machine as is, is really not all that worthwhile.

Like I said, diminishing returns has nothing to do with cost. Diminishing returns, in itself, means that your benefit keeps lessening as you add additional units of input (in this case video cards) and at some point it will either reach 0 or you will have a disutility.

An example of this could be as follows:

You start with a single video card which gives you X performance. You add a second one and you get 1.8x performance. You add an extra pair (so you have 4) and you get 3x performance.

This implies that the marginal utility from adding the second card is .8x and the marginal utility for each of the third and fourth cards is .6x. From this you could deduct, with reasonable certainty, that the returns will continue to diminish until they reach zero and you start getting disutility as you add more and more cards. This is diminishing returns.

Regardless, system performance WILL experience diminishing returns because returns ALWAYS diminish in PCs because at the same time you are increasing the number of consumers (in this case video cards) you are splitting the rest of the resources between them. What I meant to point out is that the video subsystem is not at fault here for weak performance increase when going SLI with higher-end card, it's the CPU (and maybe memory subsystem) that will make performance drop. Some applications, in fact, will experience close to no performance diminishment.

It depends on what you are considering for your "Units of input". You are looking at it as the number of video processors you are adding to the system, while I was looking at it as the amount of money I was tossing into it.

And I agree, the video subsystem is extremely powerful, and there probably is a very good use for it, but for 99% of the people out there, they won't be running that one application that scales that well over 4 video cores without being severely bottlenecked by the CPU, etc. Perhaps if it was in a Dual-processor system with 2 hyperthread enabled dual core P4's, and running some of the newer games that actually can use more than one core you'd see some difference. Maybe.

That case painted with flame suits what is inside the case. Quad SLI mode will definitely eat a lot of power. In addition, the factory OCed P4 must be a fireball too! I feel the heat just thinking about it. It must be a good buy for winter. This flame thrower can heat up a dorm room pretty well. Ah, the case painted with flame!

"... but out of the gate a single 7800GTX out performs two 7800GT cards in SLI."

What?

"Paying an extra $500 for a computer in this environment -- same piece of hardware -- paying $500 more to get a logo on it? I think that's a more challenging proposition for the average person than it used to be." -- Steve Ballmer